Engineers design earthquake safety

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Last week, engineers designing the Giant Magellan Telescope announced that they have solved a huge design challenge. Specifically, protecting a 22-story rotating observatory and seven of the world’s largest monolithic mirrors from earthquake damage. The innovative seismic protection design received top marks from an independent review panel of international experts in early November, paving the way for the next generation of observatory design.

“The structures of the next generation of extremely large telescopes are so massive, their instruments so sensitive, and the seismic environments they are in are so intense that there really is no way to avoid seismic protection. We need a seismic isolation system to keep the telescope operational, ”said Dr Bruce Bigelow, site, enclosure and facilities manager for the Giant Magellan Telescope in a statement.

The Giant Magellan Telescope is a new 30-meter ground-based telescope under construction at the Las Campanas Observatory in Chile’s Atacama Desert, one of the best places on Earth to observe the universe. But while this remote region boasts over 300 clear central galactic nights per year, it is also home to some of the largest, most frequent and most destructive earthquakes on record. Large earthquakes in Chile can last longer than three minutes and often exceed seven on the surface wave magnitude scale (MS).

Overcome the obstacles

Bigelow told IndustryWeek that the biggest challenge in accomplishing this feat was meeting the seismic protection requirements in a way that still allowed the telescope to function normally. “Seismic isolation systems for ordinary buildings aim to protect occupants, and most buildings are generally not dynamic structures per se. The telescope, on the other hand, must be able to move at two angles (azimuth and elevation) and must be able to perform these movements in a routine, precise and continuous manner as the telescope tracks objects in the sky, ”he says. “Therefore, the SIS must be stiff enough to withstand the telescope’s normal tracking reactions (up to a certain threshold), and then smoothly transition to a flexible state (above the threshold), to protect the telescope from movement. important soil. ”

According to Bigelow, “The key to the GMT Seismic Isolation System is the use of a unique arrangement of commercially available seismic bearings (single friction pendulums), which have the rigidity and frictional properties necessary to support operation. normal telescope, and a smooth transition. in an isolation function above a certain level of ground movement.

A secondary, but also critical aspect of the design is the “Pier Refocusing and Monitoring System” (PRMS), Bigelow explains. “The PRMS is a hydraulic system made up of hydraulic cylinders, sensors and a control system, which provides the forces necessary to accurately reposition the telescope and jetty (a combined mass of about 6,200 metric tons, or about £ 14million) for the ‘home’ position after a big earthquake, ”he says.

Dig deeper

The Seismic Protection System – also known as the Seismic Isolation System – of the Giant Magellan Telescope is unprecedented in the telescope world, in terms of size and complexity. Unlike hospitals or large bridges, the seismic isolation system must not only protect the structures from collapse, but also prevent the structure and fragile optical components inside from needing repair. Because the Giant Magellanic Telescope’s seismic isolation system serves as the basis of the telescope, it must be very reliable. By design, the probability of failure of the seismic insulation is less than 0.5% over the 50-year life of the observatory. The system is designed to remain inactive during small “harmful” earthquakes that are common at Las Campanas Observatory. The system will only activate during extreme earthquakes which will typically occur on a time scale of 1 to 2 years.

The Giant Magellan Telescope’s seismic isolation system consists of two lines of defense that keep it secure and allow it to return to operations within hours or weeks, depending on the magnitude of a seismic event.

1. Seismic Isolation System: A circular array of 24 single friction pendulum isolators that support the telescope and its jetty and protect the telescope’s optical components and instruments from active ground movement caused by a major earthquake.

2. Jetty Refocusing System: A hydraulic system that can return the telescope to its original resting and operating position following a major earthquake.

After a major earthquake, the friction pendulum insulators may not return the telescope exactly to its normal operating position. “The isolation system will return the telescope to its ‘original’ position in a few inches, but that’s not enough,” Bigelow said in a statement. “This is where the hydraulic system of the jetty re-centering system comes in, which can move the 6,000 metric tons of the telescope and jetty and bring the telescope back to a fraction of an inch from where it stood. before the earthquake. ”


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